Pulsed NMR instruments measuring spin-lattice relaxation time of hydrogen protons of interstitial fluids within samples of porous media are well known but have been principally confined to a laboratory environment. For example, in "Pulsed Nuclear Magnetic Resonance Studies of Porosity, Movable Fluids and Permeability of Sandstones", Journal of Petroleum Technology, June 1969, A. Timur indicates flow properties of rock samples can be deduced from NMR responses if measured by a laboratory NMR instrument. In providing such result, the measuring steps also include allowing the nuclei of the interstitial fluid to return to equilibrium each time a response is sought. Then the sample is subjected to a 180.degree. RF pulse before the 90.degree. RF pulse is applied. Total time required per NMR response: about 4 seconds.
Although the results obtained by the above laboratory NMR apparatus are and remain impressive (even though less efficiently provided), the fact that the rock samples have to be sent to the laboratory from the field for analysis is a drawback. Reason: The instrument is essentially immovable because of the DC magnet weight requirement for generating the homogeneous DC field of interest. Furthermore, within the oil industry, sophisticated pulsed NMR instruments also require (in addition to large homogeneous DC fields) complex control and pulsing circuitry, such circuitry being required to generate RF magnetic fields along axes parallel as well as perpendicular to the DC field of interest (i.e., requires both the use of 90.degree. and 180.degree. RF fields in successive pairs for realignment and reorientation purposes). In this regard, see U.S. Pat. No. 4,166,972 for "NMR Discrimination Apparatus and Method Therefor", J. D. King et al., in which the use of pairs of RF magnetic fields for such purposes are described in detail, in testing environments other than the oil industry.
Circuitry to detect resulting NMR signals can also be extremely complex and costly to manufacture and moreover, in some cases, have been found to be also subject to large background phasal incoherence. Result: In some cases the NMR responses of interest are swamped by the inherent background noise generated by the control and magnetic field generating circuity.